1. Field of the Invention
The present invention relates to a liquid crystal display using ferroelectric liquid crystal to perform tonal displays.
2. Related Background Art
As display elements using ferroelectric liquid crystal (FLC), there has hitherto been known an element such as disclosed in Japanese Patent Laid-Open Application No. 61-94023, wherein ferroelectric liquid crystals are injected into the orientationally processed liquid crystal cells having two glass substrates oppositely arranged with a cell gap of 1 to 3 .mu.m therebetween and transparent electrodes being formed on the opposite faces thereof.
The above-mentioned display element using ferroelectric liquid crystal is characterized in that with the spontaneous polarization of the ferroelectric liquid crystal, this element can utilize the coupling force between the outer electric field and the spontaneous polarization for switching, and that it is possible to perform switching by the application of the outer electric field because the major axial direction of the ferroelectric liquid crystals corresponds to the polarization direction of the spontaneous polarization one to one.
As a ferroelectric liquid crystal, chiral smectic liquid crystal (SmC*, SmH*) is generally used. This liquid crystal presents the torsional orientation for the major axes of the liquid crystal molecules in bulk, but by placing this liquid crystal in the cell gap of approximately 1 to 3 .mu.m as described above, it is possible to eliminate such a torsion given to the major axes of the liquid crystal molecules (P213-P234 N. A. CLARK et al, MCLC. 1983, Vol 94).
The ferroelectric liquid crystal is mainly used for binary (black and white) display elements by enabling the two stabilized states to be light transmitting and shielding conditions. It is also possible to use the ferroelectric liquid crystal for a multivalue display, that is, an intermediate tonal representation. One of the intermediate tonal display methods is such that an intermediate light transmitting condition is produced by controlling the area ratio of bistable condition in pixels. Hereinafter, this method (area modulation method) will be described in detail.
FIG. 5 shows the relation between the switching pulse amplitude and transmittivity of a ferroelectric liquid crystal element, and is a graph plotting it with the amount of the transmitting light I as function of the amplitude V of the single pulse obtained after having applied a single pulse of one-way polarity to the cell (element) in a totally shielded state (black). When the pulse amplitude is less than the threshold value V.sub.th (V&lt;V.sub.th), the amount of the transmitted light will not vary. The transmitting state of the pixels after the application of pulse is not different as shown in FIG. 6B from the state of the pixels before the application thereof as shown in FIG. 6A. When the pulse amplitude V exceeds the threshold value, portions of the pixels (V.sub.th &lt;V&lt;V.sub.sat) change to the other stable state, that is, the light transmitting condition represented in FIG. 6C, and an intermediate amount of transmitting light is shown as a whole. Accordingly, if the pulse amplitude V becomes great enough to exceed the saturation value V.sub.sat (V.sub.sat &lt;V), the amount of light reaches a constant value because the entire pixel become light transmittable as shown in FIG. 6D.
Thus, the area modulation method is to represent intermediate tones by controlling voltage so as to enable the pulse amplitude V to be V.sub.th &lt;V&lt;V.sub.sat.
However, with a simple driving method such as this, there is still room for improvement, as set forth below.
The relation between a voltage V and an amount of transmitted light I shown in FIG. 5 depends on the cell thicknesses and temperatures. Accordingly, there takes place an event that different tonal levels are represented for applied pulses of a same voltage amplitude if distributions of cell thicknesses and temperatures are present in the display panel.
FIG. 7 is a view for explaining this event, and it is a graph showing the relation between the voltage amplitude V and the amount Of transmitting light I as in FIG. 5, but there are shown two curved lines: a curved line H which shows the relation at high temperatures and a curved line L which shows the relation at low temperatures. In other words, in a display (display element) having a large display size, the temperature distribution often occurs in the same panel (display portion). Therefore, even if the representation of an intermediate tone is attempted at a certain voltage V.sub.ap, there are some cases that uniform display cannot be obtained because the intermediate tonal level becomes uneven over an area from the amount of transmitting light I.sub.1 to that of I.sub.2 as shown in FIG. 7.
Now, with a view to solving this, a four-pulse method is designed by the inventor hereof as proposed in Japanese Patent Gazette No. 4-218022. As shown in FIG. 8, this driving method is to obtain an equally reversed area ultimately by applying a plurality of pulses A to D to the low threshold portion and high threshold portion on a same scanning line in the panel. Hereinafter, the description will be made of the four-pulse method in conjunction with an area tonal method which controls the domain area of black and white in pixels. However, the four-pulse method itself is fundamentally a driving method to be used commonly for the elements thereby to modulate the transmittivity of pixels by the application of a voltage or by means of pulse widths. For example, therefore, this method is applicable as a light amount adjustment method to the chiral smectic phase C having the orientation of spiral pitches of less than the wavelength of light, a short spiral of less than 0.7 .mu.m, for example, because the method can be used in an orientational mode where the amounts of transmitted light vary without the domain walls to be formed in pixels.
Nevertheless, there is still room for improvement in the foregoing four-pulse method as set forth below.
Firstly, as shown in FIG 8,. according to the four-pulse method, a pulse A is applied at first to the pixels on a selected scanning line. Then, the pulses B, C, and D are applied sequentially. At this juncture, however, the write pulses A, B, C, and D to be applied are affected respectively by the preceding pulses. Consequently, due to the voltage of the preceding pulse, the voltage (threshold value) required to reverse the liquid crystal is slightly different when the following pulse is to be applied. A phenomenon of this kind hinders setting of the voltage value of a pulse B. When the variation of the threshold value due to the presence of a preceding pulse is small, it may be possible to accept it as an allowable error (even in such a case, the accuracy of the tonal representation is lowered). However, if the variation is great, it becomes impossible to use the four-pulse method itself. This is due to the fact that the four-pulse method is operative on the assumption that the four pulses are of an equal value when applied.
Secondly, the pulse A in FIG. 8 is a resetting pulse and there is no problem because a voltage which exceeds the threshold value is applicable. However, for the other pulses B, C, and D, it is necessary to provide domain walls i, j, and k in the pixels. To each of them, a voltage extremely close to the threshold value is applied. When a switching is conducted with a voltage which is extremely close to the threshold value for liquid crystal molecule but not sufficiently above the threshold value, the position of the domain walls is significantly affected by the pulse applied immediately preceding thereto. Such an effect of the immediately preceding voltage as this is not so serious a problem when the variation of the voltage value is small. However, if the variation is great, some improvements are required.
Thirdly, such an effect as this can also be produced by a voltage immediately after writing. As shown in FIG. 8, even if the domain wall j is set up by the pulse C, for example, the position of the domain wall j will be shifted by the proceeding pulse D if it has a voltage which is greater than a certain value. In other words, a write pulse is easily affected by the cross talk from the following pulse. This is a point which should be improved.
Now, fourthly, even when the effects produced by the variation of the threshold value and cross talk as described in the preceding paragraphs 1 to 3 are not so great, the number of write pulses is many as compared with the methods described in conjunction with FIGS. 5 and 6A to 6D. In other words, in the methods shown in FIGS. 5 and 6A to 6D require only pulses A and B in FIG. 8, but in the four-pulse method, pulses C and D are further required. This means that the time (frame time) required to write the entire surface of the panel is prolonged that much. As a result, if the entire image plane should be written all the time, the quality of display is affected, not to mention the display of animated representations, and in the worst case, no representation is possible except still images.
As described above, the four-pulse method itself has the foregoing first to third factors to result in errors and the fourth problem of delay in displaying velocity.